The present invention relates generally to voice and data call prioritization in a mobile communication system, and more particularly to a method and apparatus for prioritizing voice and data calls within a Code Division Multiple Access (CDMA) mobile telephony system which allows an enhanced Radio Frequency (RF) loss reduction for voice calls.
This invention relates to the protection of voice calls in a wireless cellular telecommunications system where voice and data services are permitted. More specifically the voice protection technology may be applied in spread spectrum, or code division multiple access (CDMA) systems operating according to Telecommunications Industry Association standard specified as xe2x80x9cIS-95B.xe2x80x9d Furthermore, the present invention may also be applied to any Third Generation Partnership Projects (3GPP) Universal Mobile Telecommunications System (UMTS) specification.
In a typical CDMA environment, unique digital codes, rather than RF frequencies, are used to differentiate subscribers. During a call, a user is assigned a binary, Direct Sequence code, and that users""0 information data stream is impressed upon a much-higher-bit-rate data stream. The digital codes are shared by both the mobile station (cellular phone) and the base station, and are called xe2x80x9cpseudo-Random Code Sequences.xe2x80x9d0 All users, therefore, are able to share the same range of radio spectrum. Each of the coded information signals overlaps all other coded signals and all signals are transmitted as modulations of radio frequency carrier waves, jointly received as a composite signal at a receiver. By utilizing the shared digital code at the receiver, the composite signal can be isolated and decoded.
In order to reduce interference between different call connections and to maintain sufficient communication link quality and data throughput, transmission power levels may be manipulated at both the base stations and the mobile stations. Preferably, the transmission power levels operate at the minimum level in order to maintain satisfactory mobile service and maximize system capacity. Reverse link power control varies the power level of the reverse communications link (mobile unit to base station), while forward-link power control varies the power level of the forward communications link (base station to mobile unit).
As the number of users transmitting information increases, the interference between the users"" signals also increases and transmission quality suffers as a result. The interference associated with increased cell usage within a CDMA system can be easily understood by analogy to a cocktail party. CDMA technology can be likened to a large, single room with a large number of people, in pairs, who wish to carry on a conversation. The people in the room only wish to talk to each other, and are uninterested in the conversations of the others in the room. If each pair in the room speaks a language that only they can understand, each pair is able to carry on a conversation while experiencing little or no interference from the other conversations. The analogy is that the air in the room is a wideband carrier, and the languages are represented by the xe2x80x9ccodesxe2x80x9d0 assigned by the CDMA system.
As the number of people carrying on a conversation in the room increases, the background noise in the room (interference from other users) also increases, making it more difficult for the pairs to understand each other (frame erasure rates get too high). Likewise, as the distance between the pairs increases, the more likely it is that the background noise of the room will interfere with the pairs conversation. By controlling the voice volume (signal strength) of all the participants to a level no more than necessary, the number of conversations which can take place in the room can be maximized.
In such systems where voice and data signals can be simultaneously transmitted over the air on the same channels, forward link power control typically uses forward link quality information feedback transmitted on reverse links to update forward link power levels. This feedback information can be in the form of bits used to indicate whether an individual forward link traffic channel frame was erased. This information can also be in the form of a transmitted message used to indicate that some number of forward link frames were erased during a given time span. Reverse channel power control currently uses a method in communication systems, such as a CDMA system, whereby a power control group is transmitted from the mobile communication unit and received by the base station. The base station compares the energy of the power control group to a set point threshold and instructs the mobile communication unit on a forward link to power up or power down as necessary by transmitting a power adjustment command to the mobile or remote unit.
Under normal conditions, this type of closed loop power control will result in sufficient power levels that maintain an interference or noise level of the receive signal at a substantially fixed and acceptable level. However, a call initiated by a remote high speed data (HSD) user within a given sector or cell causes interference that can deteriorate signal quality for other users including both voice and data calls occurring within the cell and neighboring cells. These other users typically operate at minimum necessary power levels. The HSD user transmits at a significantly higher power level. When the cell is at or near maximum capacity (overload condition), the high speed data transmission interferes with the other users in the cell, and especially impacts the voice transmissions of users who are furthest away from the BTS. This interference can result in unacceptable lost voice call and/or degraded service quality.
Current congestion control technology allows for the degradation in the frame erasure rate (FER) of all transmission signals or pilot carrier signal power when the linear power amplifier (LPA) is at maximum allowable power. Other congestion control implementations limit the number of available supplemental channels, or alternatively, the data rates for supplemental channels based on cell load criteria, (i.e., total forward power with respect to LPA allotment). These methods do not allow for a single mobile unit to trigger congestion relief, and therefore, both data and voice users are given equal preference after receiving a transmission channel. The net result of these congestion control technologies is that a few data users can monopolize a cell and effectively lock out voice users, or degrade voice user service so as to cause the voice call to be dropped, ultimately resulting in user dissatisfaction.
Thus there is a need for a congestion control technology which protects voice calls that are in imminent danger of being dropped by reducing the amount of interference to the voice user by the data user.